Bases of Igneous Rock Classification

• Mineral Content:
  • Principal minerals: Silicate minerals (Bowen’s Series).
  • Felsic minerals (quartz, plagioclase, alkali feldspar, muscovite).
  • Mafic minerals (biotite, hornblende, pyroxene, olivine).
• Chemical Composition:
  • Silicic (SiO₂), Magnesian (MgO), Alkaline (Na₂O + K₂O), Aluminous (Al₂O₃).
  • Acidic (> 66% SiO₂), Intermediate (52–66% SiO₂), Basic (45–52% SiO₂), Ultrabasic (< 45% SiO₂).
• Color:
  • Leucocratic: Light-colored rocks.
  • Melanocratic: Dark-colored rocks.
  • Color index (M): Percentage of dark minerals.

Issues with Terminology

• Felsic/Mafic:
  • Terms are based on mineral composition but often used based on color.
  • Example: Dark-colored quartz and feldspars are still felsic, leading to confusion.
• Acidic/Basic:
  • Terms derived from outdated concepts (silicic acid in solution).
  • Not accurate for describing magmas or modern petrology.
  • Basic rocks are generally mafic, but chemical content should take precedence over color.

Terms in Literature

• Mafic and Felsic:
  • Applied to both rocks and magmas.
  • Considered appropriate despite being mineral-based.
• Basic and Acidic:
  • Applied to magmas and based on chemical composition rather than mineralogy.
• Other terms:
  • Descriptive terms like “basaltic magma” and “andesitic magma” name the magma based on its solidified equivalent.
  • Use of precise chemical descriptions (e.g., “silicic” or “alkaline”) for unusual concentrations of certain components.
• Terminological inconsistencies can lead to confusion in the classification and description of igneous rocks and magmas.
• While terms like “felsic” and “mafic” dominate North American literature, British geologists prefer “basic” and “acidic,” focusing more on chemical composition.
• A balance between mineral-based and chemical-based descriptions is essential for clarity in igneous rock classification.

The IUGS Classification

• This Classification was developed by the International Union of Geological Sciences (IUGS) to standardize igneous rock nomenclature.

• The IUGS system requires that we determine the mineral components of a rock and plot the percentages of three of those components on appropriate triangular diagrams to determine the proper name.

Plotting on a Triangular Diagram

Steps to Plot 70% X, 20% Y, and 10% Z

1. Understand the Axes of the Triangular Diagram

• Each apex represents 100% of a component:
  • X at the top apex.
  • Y at the bottom right.
  • Z at the bottom left.
• Percentages decrease from each apex to the base opposite the apex.
  • The sum of X, Y, and Z must equal 100%.

2. Plot the 70% X Value

• Find the X-axis:
  • 70% X is represented by counting 7 divisions (70%) from the Y–Z base toward the X apex.

3. Plot the 20% Y Value

• Move along the Y-Z base:
  • Starting from the Z apex, move 67% of the way toward Y (because Y/(Y + Z) = 20/30 = 67%).

4. Locate the Point

• Draw a line from the 67% Y-Z point toward the X apex.
• Mark the point where this line intersects the 70% X line.

5. Verify the Z Value

• The point should also align with 10% Z by counting up 1 division from the X–Y base toward the Z apex.
• The intersection of these lines represents 70% X, 20% Y, and 10% Z.

Final Result

• The point A representing 70% X, 20% Y, and 10% Z is plotted where the 70% X line, the 10% Z line, and the 20% Y line intersect.

Classification Based on Mineral Content

• For most igneous rocks: Q + P + A (Quartz, Plagioclase, Alkali Feldspar).
• If feldspathoids are present: F + P + A (Feldspathoids, Plagioclase, Alkali Feldspar).

Classification of The Plutonic Igneous Rocks

Classification of Volcanic Rocks

Chemical Classification of Volcanic Rocks

Classification of Pyroclastic Rocks

• Pyroclastic Rocks: Formed from volcanic fragments (pyroclasts) during explosive eruptions.
• Classified based on:
  • Type of fragmental material.
  • Size of the fragments.
  • Chemical or modal composition (if possible, when impurities are minimal).

Fragment Size Classification

• > 64 mm: Bombs (if molten during fragmentation—thus typically rounded/blobby, flattened, or stretched) or Blocks (if not molten during fragmentation—thus typically angular or broken).
• 2–64 mm: Lapilli.
• < 2 mm: Ash.

Key IUGS Requirements

• A true pyroclastic rock must contain at least 75% pyroclasts.
  • Pyroclasts can include individual crystals, glass, or rock fragments.
  • Crystals are referred to as crystal fragments, not phenocrysts.

Types of Pyroclastic Rocks

• Tuffs: Classified based on fragment type.
  • Coarse Ash Tuff: Fragments 1/16 mm to 2 mm.
  • Fine Ash Tuff (Dust Tuff): Fragments < 1/16 mm.
  • Lithic Tuff: Dominated by rock fragments.
  • Vitric Tuff: Dominated by pumice/glass fragments.
  • Crystal Tuff: Dominated by crystal fragments.
• Example: Rhyolitic Lapilli Tuff - A pyroclastic rock dominated by lapilli-sized fragments with a rhyolitic composition.

Mixed Pyroclastic and Sedimentary Rocks

• Tuffite: Contains both pyroclastic and sedimentary clastic material (epiclasts).
  • Can be further classified (e.g., Tuffaceous Sandstone).
• Epivolcaniclastics: Secondary deposits, not directly from eruptions (e.g., volcanic mudflows or reworked epiclasts).

Special Types of Pyroclastic Deposits

• Aquagene Tuff: Waterborne ash accumulation, either from subaqueous eruptions or reworked by water.
• Hyalotuff/Hyaloclastite: Created when magma is shattered upon contact with water.

Other Ignous Rocks